U.S. patent number 5,849,071 [Application Number 08/876,304] was granted by the patent office on 1998-12-15 for liquid source formation of thin films using hexamethyl-disilazane.
This patent grant is currently assigned to Matsushita Electronics Corporation, Symetrix Corporation. Invention is credited to Gary F. Derbenwick, Shinichiro Hayashi, Larry D. McMillan, Carlos A. Paz de Araujo, Michael C. Scott, Narayan Solayappan.
United States Patent |
5,849,071 |
Derbenwick , et al. |
December 15, 1998 |
**Please see images for:
( Certificate of Correction ) ** |
Liquid source formation of thin films using
hexamethyl-disilazane
Abstract
A precursor liquid comprising several metal 2-ethylhexanoates,
such as strontium, tantalum and bismuth 2-ethylhexanoates, in a
solvent such as xylenes/methyl ethyl ketone and a small amount of
hexamethyl-disilazane. The liquid is dried, baked, and annealed to
form a thin film of a layered superlattice material, such as
strontium bismuth tantalate, on the substrate.
Inventors: |
Derbenwick; Gary F. (Colorado
Springs, CO), McMillan; Larry D. (Colorado Springs, CO),
Solayappan; Narayan (Colorado Springs, CO), Scott; Michael
C. (Colorado Springs, CO), Paz de Araujo; Carlos A.
(Colorado Springs, CO), Hayashi; Shinichiro (Colorado
Springs, CO) |
Assignee: |
Symetrix Corporation (Colorado
Springs, CO)
Matsushita Electronics Corporation (JP)
|
Family
ID: |
24871400 |
Appl.
No.: |
08/876,304 |
Filed: |
June 16, 1997 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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714774 |
Sep 16, 1996 |
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Current U.S.
Class: |
106/287.11;
257/E21.272 |
Current CPC
Class: |
H01L
21/02222 (20130101); C23C 18/1216 (20130101); H01L
21/02197 (20130101); H01L 21/022 (20130101); H01L
21/31691 (20130101); H01L 21/02282 (20130101); C23C
18/1225 (20130101) |
Current International
Class: |
C23C
18/00 (20060101); C23C 18/12 (20060101); H01L
21/02 (20060101); H01L 21/316 (20060101); H01L
21/314 (20060101); C09D 007/12 (); C09K
015/32 () |
Field of
Search: |
;106/287.11 |
References Cited
[Referenced By]
U.S. Patent Documents
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5423285 |
June 1995 |
Paz De Araujo et al. |
5429673 |
July 1995 |
Peterson et al. |
5456945 |
October 1995 |
McMillian et al. |
|
Primary Examiner: Brunsman; David
Attorney, Agent or Firm: Duft, Graziano & Forest,
P.C.
Parent Case Text
This application is a division of application Ser. No. 08/714,774,
filed 16 Sep. 1996.
Claims
We claim:
1. A liquid precursor for forming a metal oxide, said precursor
comprising: a plurality of metal moieties in effective amounts for
forming a layered superlattice material upon application said
precursor to a substrate and heating; and a solvent comprising
hexamethyl-disilazane.
2. A liquid precursor as in claim 1 wherein said solvent further
includes a liquid selected from the group consisting of methyl
ethyl ketone, isopropanal, methanol, tetrahydrofuran, xylene,
n-butyl acetate, octane and 2-methoxyethanol.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to a method of formation of thin films using
liquid sources, and more particularly to the fabrication of thin
films of metal-oxides of suitable thinness and quality of use in
integrated circuits.
2. Statement of the Problem
It is known that liquid deposition processes, such as the process
of misted deposition of a liquid precursor as described in U.S.
Pat. No. 5,456,945 issued Oct. 10, 1995 and the spin-on process
described in U.S. Pat. No. 5,423,285 issued Jun. 13, 1995, are
useful in making integrated-circuit-quality thin films. It is also
known that the misted deposition process has important advantages
for the routine manufacture of integrated circuits. While the
misted deposition process could be used to make good,
integrated-circuit-quality, thin films of barium strontium titanate
and other relatively simple metal oxides, when the process was used
with more complex materials, such as the layered superlattice
materials, high-quality films could be made only if the shapes of
the thin film layers involved only flat structures, such as flat,
uniform dielectric layers in capacitors. When the thin film
structures involved sharp corners, such as in steps, the layered
superlattice materials tended to fill the corners and not follow
the contour of the underlying layers. In the integrated circuit
art, this is expressed as not having good step coverage when used
for layered superlattice materials. If the viscosity of the liquid
source was adjusted to give better step coverage, then the quality
of the film declined significantly, resulting in shorted layers and
relatively poor electronic properties. However, state-of-the-art
integrated circuits involve quite complex structures involving
steps and other sharp corners. Thus, up to now, the use of the
misted deposition process for layered superlattice materials has
been limited. Since the layered superlattice materials have such
extraordinary properties in integrated circuits, it would be highly
desirable to have a misted deposition process that would allow the
materials to be used in integrated circuits having complex
structures.
SUMMARY OF THE INVENTION
The invention solves the above problems by providing
hexamethyl-disilazane (HMDS) as a solvent in the liquid precursor
used to deposit metal compounds. It has been found that the HMDS
makes a substantial improvement in the step coverage resulting from
the misted deposition of the layered superlattice materials. It has
also been found to improve the step coverage of other metal oxides
deposited in both the misted deposition process and the spin-on
process, though the improvements are not as dramatic as those for
the layered superlattice materials in the misted deposition
process.
The invention provides a method of fabricating an integrated
circuit including a thin film of a metal compound, the method
comprising the steps of: providing an integrated circuit substrate;
providing a liquid precursor including: at least one metal in
effective amounts for forming a desired compound including the
metal; and hexamethyl-disilazane; applying the liquid precursor to
the substrate; treating the liquid layer deposited on the substrate
to form a solid film of the desired metal compound; and completing
the fabrication of the integrated circuit to include at least a
portion of the metal compound in the electrical component of the
integrated circuit. Preferably, the metal compound comprises a
layered superlattice material. Preferably, the layered superlattice
material comprises a material selected from the group consisting of
strontium bismuth tantalate, strontium bismuth niobate, and
strontium bismuth tantalum niobate. Preferably, the step of
applying comprises: placing the substrate inside an enclosed
deposition chamber; producing a mist of the liquid precursor; and
flowing the mist through the deposition chamber to form a layer of
the precursor liquid on the substrate. Preferably, the step of
flowing is performed while maintaining the deposition chamber at
ambient temperature. Preferably, the step of providing a precursor
includes the step of adding an initiator having a boiling point
between 50.degree. C. and 100.degree. C. to the precursor prior to
the step of producing a mist. Preferably, the initiator comprises a
solvent selected from the group consisting of methyl ethyl ketone,
isopropanal, methanol, and tetrahydrofuran. Alternatively, the step
of applying comprises using a spin-on process to apply the
precursor to the substrate. Preferably, the liquid precursor
comprises a solvent and a metal compound selected from the group
consisting of metal alkoxides and metal carboxylates. Preferably,
the step of treating includes one or more steps from the group of
drying, baking and annealing the layer deposited on the substrate.
Preferably, the liquid precursor includes a compound of the metal
in a solvent, the solvent selected from the group consisting of
xylene, n-butyl acetate, and 2-methoxyethanol. Preferably, the
metal includes a metal selected from the group consisting of
strontium, calcium, barium, bismuth, cadmium, lead, titanium,
tantalum, hafnium, tungsten, niobium, zirconium, scandium, yttrium,
lanthanum, antimony, chromium, and thallium.
In another aspect the invention provides a liquid precursor for
forming a metal oxide, the precursor comprising: a plurality of
metal moieties in effective amounts for forming a layered
superlattice material upon application the precursor to a substrate
and heating; and a solvent comprising hexamethyl-disilazane.
Preferably, the solvent further includes a liquid selected from the
group consisting of methyl ethyl ketone, isopropanal, methanol,
tetrahydrofuran, xylene, n-butyl acetate, octane and
2-methoxyethanol.
In a further aspect, the invention provides a method of fabricating
a thin film of a layered superlattice material, the method
comprising the steps of: providing a liquid precursor including: a
plurality of metal moieties in effective amounts for forming a
layered superlattice material; and hexamethyl-disilazane; placing a
substrate inside an enclosed deposition chamber; producing a mist
of the liquid precursor; flowing the mist through the deposition
chamber to form a layer of the precursor liquid on the substrate;
and treating the liquid layer deposited on the substrate to form a
solid film of the layered superlattice material. Preferably, the
step of flowing is performed while maintaining the deposition
chamber at ambient temperature. Preferably, layered superlattice
material forms part of an electrical component in an integrated
circuit, the method further including the step completing the
fabrication of the integrated circuit to include at least a portion
of the film of the layered superlattice material in the electrical
component of the integrated circuit. Preferably, the layered
superlattice material comprises a material selected from the group
consisting of strontium bismuth tantalate, strontium bismuth
niobate, and strontium bismuth tantalum niobate. Preferably, the
metals include a metal selected from the group consisting of
strontium, calcium, barium, bismuth, cadmium, lead, titanium,
tantalum, hafnium, tungsten, niobium, zirconium, scandium, yttrium,
lanthanum, antimony, chromium, and thallium.
The addition of hexamethyl-disilazane to the layered superlattice
material precursor for the first time permits the fabrication of
integrated circuits having layered superlattice material portions
made in a misted deposition process that provides both excellent
step coverage and excellent electronic properties. In contrast to
the prior art, there is almost no variation in thickness of the
layered superlattice material as it passes over the step. This
substantial improvement in the results for the layered superlattice
materials has led to the investigation of the use of HMDS for other
materials and in other liquid deposition processes. In each case
there has been some improvement of results. Numerous other
features, objects and advantages of the invention will become
apparent from the following description when read in conjunction
with the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWING
FIG. 1 and FIG. 1A are cutaway side views of the deposition chamber
of the misted deposition apparatus used in the preferred embodiment
of the invention;
FIG. 2 is an enlarged plan view of an intake and exhaust nozzle
assembly of the apparatus of FIG. 1;
FIG. 3 is an enlarged schematic top view of a manifold system used
in the apparatus of FIG. 1;
FIG. 4 is a flow chart showing the preparation of a layered
superlattice material thin film according to the preferred
embodiment of the invention;
FIG. 5 is a drawing of an electron micrograph of an integrated
circuit device fabricated with the process of the invention showing
the step coverage of a thin film of strontium bismuth tantalate
applied to a substrate;
FIG. 6 shows a cross-sectional view of a portion of an integrated
circuit capacitor fabricated utilizing the method of the
invention;
FIG. 7 shows an cross-sectional view of a DRAM memory cell made
with a layered superlattice material;
FIG. 8 is a graph of the measured polarization as a function of
electric field for a strontium bismuth tantalate capacitor made
according to the process of the invention;
FIG. 9 is a graph of the measured remnant polarization versus
number of switching cycles, i.e. a fatigue curve, for the strontium
bismuth tantalate capacitor of FIG. 8; and
FIG. 10 is a graph of the leakage current versus applied voltage
for the strontium bismuth tantalate capacitor of FIG. 8.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
1. Overview
According to a primary aspect of the present invention, a precursor
liquid of a layered superlattice material, such as strontium
bismuth tantalate, are initially prepared, a mist of the solution
is generated, flowed into a deposition chamber, and deposited in a
thin film layer or layers on a substrate disposed within the
deposition chamber. As is conventional in the art, in this
disclosure, the term "substrate" is used in a general sense where
it includes one or number of layers 5 (FIG. 6) of material on which
the layered superlattice material may be deposited, and also in a
particular sense in which it refers to a silicon wafer 622 on which
the other layers are formed. Unless otherwise indicated it means
any object on which a layer of a layered superlattice material is
deposited using the process and apparatus of the invention.
Precursor liquids include sol-gel precursor formulations, which in
general are comprised of metal-alkoxides in an alcohol solvent, and
metallorganic precursor formulations, sometimes referred to as MOD
formulations, which in general comprise a metal-carboxylate formed
by reacting a carboxylic acid, such as n-decanoic acid or
2-ethylhexanoic acid, with a metal or metal compound in a solvent,
combinations thereof, as well as other precursor formulations.
Whatever the precursor, the invention includes the use of
hexmethyl-disilazane as a precursor solvent or co-solvent.
The term "mist" as used herein is defined as fine drops of a liquid
carried by a gas. The term "mist" includes an aerosol, which is
generally defined as a colloidal suspension of solid or liquid
particles in a gas. The term mist also includes a vapor, a fog, as
well as other nebulized suspensions of the precursor solution in a
gas. Since the above terms have arisen from popular usage, the
definitions are not precise, overlap, and may be used differently
by different authors. Herein, the term aerosol is intended to
include all the suspensions included in the text Aerosol Science
and Technology, by Parker C. Reist, McGraw-Hill, Inc., New York,
1983, which is hereby incorporated by reference. The term "mist" as
used herein is intended to be broader than the term aerosol, and
includes suspensions that may not be included under the term
aerosol, vapor, or fog.
As discussed in the patents referenced above, the use of precursor
liquids results in high quality of thin films because the precursor
liquid can be accurately and consistently produced such that the
desired chemical compound after deposition, is uniformly,
stoichiometrically correct and because the deposition methods of
the present invention do not involve violent chemical or physical
reactions which either significantly destabilize the chemical
compound of its predetermined molecular formulation or cause
non-uniform deposition of the compound, cracking, etc. The misted
deposition process also lends itself to large scale manufacturing
of integrated circuits because it can consistently be reproduced
and/or repeated for large numbers of wafers, and can be scaled to
the size necessary for manufacturing of large numbers of wafers. As
will be discussed in detail below, the use of hexamethyl-disilazane
as a solvent or cosolvent results in excellent step coverage and
excellent electronic properties for complex materials, such as the
layered superlattice materials.
Layered superlattice materials are described in detail in U.S. Pat.
No. 5,423,285 issued Jun. 13, 1995, and U.S. Pat. No. 5,519,234
issued May 21, 1996. In general, a layered superlattice material is
defined as a material that can be described by a single chemical
formula and which spontaneously forms itself into alternating
layers having distinctly different crystalline structure. For
example, strontium bismuth tantalate (SrBi.sub.2 Ta.sub.2 O.sub.9)
can be considered to be formed of alternating layers of a crystal
structure similar to Bi.sub.2 O.sub.3 and a crystal structure
similar to SrTa.sub.2 O.sub.6, although it must be kept in mind
that SrTa.sub.2 O.sub.6 has a tungsten bronze structure by itself,
but within the layered material it has a Perovskite structure. Thus
the layered structure is in reality a superlattice in which the
structures of the individual sublattices of the Perovskite layers
and the non-Perovskite layers are interdependent. These layered
materials are natural superlattices, as compared to other
superlattices, such as compositional superlattices, which are
manufactured or forced superlattices. Thus, the term "layered
superlattice material" is selected to distinguish these
superlattice materials from alloy type superlattice materials,
which are not layered, and superlattice heterostructures, i.e. the
compositional superlattices, which are inherently not a "material"
but rather layered structures made of at least two different
materials having different chemical formulae.
The layered superlattice materials made by the process of the
invention are polycrystalline. In the polycrystalline state, the
structure of the materials includes grain boundaries, point
defects, dislocation loops and other microstructure defects.
However, within each grain, the structure is predominately
repeatable units containing one or more ferroelectric layers and
one or more intermediate non-ferroelectric layers spontaneously
linked in an interdependent manner. Thus the layered superlattice
materials of the invention which are ferroelectric can be defined
as: (A) a material having a localized structure, within a grain or
other larger or smaller unit, which localized structure contains
predominately repeatable units containing one or more ferroelectric
layers and one or more intermediate non-ferroelectric layers
spontaneously linked in an interdependent manner. The invention
also includes materials that are not ferroelectric, and those that
include Perovskite-like layers can be included in the following
definition: (B) a material having a localized structure, within a
grain or other larger or smaller unit, which localized structure
contains predominately repeatable units containing one or more
Perovskite-like layers and one or more intermediate
non-Perovskite-like layers spontaneously linked in an
interdependent manner.
The layered superlattice materials include layered Perovskite-like
materials catalogued by Smolenskii et al. in Ferroelectrics and
Related Materials, ISSN 0275-9608, (V.3 of the series
Ferroelectrics and Related Phenomena, 1984) edited by G. A.
Smolenskii, Sections 15.3-15.7 and include:
(I) compounds having the formula A.sub.m-1 Bi.sub.2 M.sub.m
O.sub.3m+3, where A=Bi.sup.3+, Ba.sup.2+, Sr.sup.2+, Ca.sup.2+,
Pb.sup.2+, K.sup.+, Na.sup.+ and other ions of comparable size, and
M=Ti.sup.4+, Nb.sup.5+, Ta.sup.5+, Mo.sup.6+, W.sup.6+, Fe.sup.3+
and other ions that occupy oxygen octahedral; this group includes
bismuth titanate, Bi.sub.4 Ti.sub.3 0.sub.12 ;
(II) compounds having the formula A.sub.m+1 M.sub.m O.sub.3m+1,
including compounds such as strontium titanates Sr.sub.2 TiO.sub.4,
Sr.sub.3 Ti.sub.2 O.sub.7 and Sr.sub.4 Ti.sub.3 O.sub.10 ; and
(III) compounds having the formula A.sub.m M.sub.m O.sub.3m+2,
including compounds such as Sr.sub.2 Nb.sub.2 O.sub.7, La.sub.2
Ti.sub.2 O.sub.7, Sr.sub.5 TiNb.sub.4 O.sub.17, and Sr.sub.6
Ti.sub.2 Nb.sub.4 O.sub.20. It is noted that in the case of
Sr.sub.2 Nb.sub.2 O.sub.7 and La.sub.2 Ti.sub.2 O.sub.7 the formula
needs to be doubled to make them agree with the general formula.
Layered superlattice materials include all of the above materials
plus combinations and solid solutions of these materials.
Layered superlattice materials may be summarized more generally
under the formula:
where A1, A2 . . . Aj represent A-site elements in the
Perovskite-like structure, which may be elements such as strontium,
calcium, barium, bismuth, cadmium, lead, and others S1, S2 . . . Sk
represent superlattice generator elements, which usually is
bismuth, but can also be materials such as yttrium, scandium,
lanthanum, antimony, chromium, thallium, and other elements with a
valence of +3, B1, B2 . . . BI represent B-site elements in the
Perovskite-like structure, which may be elements such as titanium,
tantalum, hafnium, tungsten, niobium, zirconium, and other
elements, and Q represents an anion, which generally is oxygen but
may also be other elements, such as fluorine, chlorine and hybrids
of these elements, such as the oxyfluorides, the oxychlorides, etc.
The superscripts in formula (1) indicate the valences of the
respective elements, and the subscripts indicate the number of
moles of the material in a mole of the compound, or in terms of the
unit cell, the number of atoms of the element, on the average, in
the unit cell. The subscripts can be integer or fractional. That
is, formula (1) includes the cases where the unit cell may vary
throughout the material, e.g. in Sr.sub.0.75 Ba.sub.0.25 Bi.sub.2
Ta.sub.2 O.sub.9, on the average, 75% of the time Sr is the A-site
atom and 25% of the time Ba is the A-site atom. If there is only
one A-site element in the compound then it is represented by the
"A1" element and w2 . . . wj all equal zero. If there is only one
B-site element in the compound, then it is represented by the "B1"
element, and y2 . . . yI all equal zero, and similarly for the
superlattice generator elements. The usual case is that there is
one A-site element, one superlattice generator element, and one or
two B-site elements, although formula (1) is written in the more
general form since the invention is intended to include the cases
where either of the sites and the superlattice generator can have
multiple elements. The value of z is found from the equation:
Formula (1) includes all three of the Smolenskii type compounds:
for the type I material, w1=m-1, x1=2, y1=m, z=3m+3 and the other
subscripts equal zero; for the type II material, w1=m+1, y1=m,
z=3m,+1, and the other subscripts equal zero; for the type III
material, w1=m, y1=m, z=3m+2, and the other subscripts equal zero.
It is noted that the Smolenskii type I formula does not work for
M=Ti and m=2, while the formula (1) does work. This is because the
Smolenskii formula does not consider valences. The materials
according to the invention do not include all materials that can be
fit into formula (1), but rather only those materials that
spontaneously form layered superlattices. In summary, the materials
of the invention include all the materials as described by the
definitions (A) and (B) above, the Smolenskii formulas, and the
formula (1), plus solid solutions of all the foregoing materials.
Terms of art that have been applied to these structures include
layered perovskite-like materials, recurrent intergrowth layers,
Aurivilius materials, and self-orienting spontaneous intergrowth
layers. Even so, no one single term suffices to describe the entire
class of layered superlattice materials. Applicants have chosen the
term "layered superlattice materials" to describe the entire class
of materials because the lattices include a short range order,
e.g., a sublayer formed of a perovskite-like oxygen octahedra
lattice, and a long range order including a periodic repetition of
sublayers, e.g., a perovskite-like sublayer and a superlattice
generator metal oxide layer repeated in succession. Further, as in
other superlattice materials, the length of the periodicity can be
manipulated. For example, as is known in the art of these
materials, by adjusting the stoichiometry, the value of "m" in the
Smolenskii formulas I, II, and III above can be varied to vary the
thickness of the perovskite-like layers. See, Ferroelectrics and
Related Materials, ISSN 0275-9608, (V.3 of the series
Ferroelectrics and Related Phenomena, 1984) edited by G. A.
Smolenskii, p. 694. The dual order of these periodically repeating
structures and the ability to manipulate the periodic distances
meets the definition of a superlattice. As indicated above, the
term "layered superlattice material" should not be confused with
forced heterolattice structures that are made by sputter deposition
of successive layers. Layered superlattice materials spontaneously
form collated intergrowth layers in an anneal, and do not require
the forced deposition of successive layers.
According to the preferred embodiment of the present invention, the
mist of a precursor liquid is evenly flowed across and onto a
substrate at substantially ambient temperature. That is, unlike the
prior art, the substrate is not heated. In this disclosure the term
"ambient" means at the temperature of the surroundings, which
preferably is room temperature, which is generally between
15.degree. C. and 40.degree. C. However, because various processes
may be occurring during the deposition, such as drawing a vacuum,
electrical poling, and/or the application of ultraviolet radiation,
the actual temperature within deposition chamber 2 may vary from
the temperature of the room in which the deposition takes place.
Thus the use of the words "substantially ambient temperature".
Substantially ambient temperature means generally within the range
of -50.degree. C. and 100.degree. C. As will be discussed further
below, a key aspect of the flow process is that the mist is flowed
across the substrate via multiple input ports and exits the area
above the substrate via multiple exhaust ports, with the ports
being distributed in close proximity to and about the periphery of
the substrate to create a substantially evenly distributed flow of
mist across the substrate.
Another feature of the deposition process is that it is a
relatively low energy process as compared to prior art deposition
processes. It is believed that the deposition is caused by
relatively low energy kinetic interactions between the liquid
particles and also relatively low energy kinetic interactions
between the particles and the barrier plate opposite the substrate.
It has been found that heating the deposition chamber or substrate
during deposition leads to inferior quality thin films. During,
after, or both during and after deposition, the precursor liquid is
treated to form a thin film of solid layered superlattice material
on the substrate. In this context, "treated" means any one or a
combination of the following: exposed to vacuum, ultraviolet
radiation, electrical poling, drying, heating, and annealing. In
the preferred embodiment UV radiation and electrical poling are
optionally applied to the precursor solution during deposition. The
ultraviolet radiation is preferably also applied after deposition.
After deposition, the material deposited on the substrate, which is
liquid in the preferred embodiment, is preferably exposed to vacuum
for a period, then is heated, and then annealed. The preferred
process of the invention is one in which the misted precursor
solution is deposited directly on the substrate and the
dissociation of the organics in the precursor that do not form part
of the desired material and removal of the solvent and organics or
other fragments takes place primarily after the solution is on the
substrate. However, in another aspect the invention also
contemplates a process in which the final desired chemical compound
or an intermediate compound is separated from the solvent and
organics during the deposition and the final desired chemical
compound or an intermediate compound is deposited on the substrate.
In both aspects, preferably, one or more bonds of the precursor
pass through to the final film.
An important parameter of many complex thin films used in
integrated circuits, such as ferroelectric films, is that they are
generally required to be quite thin, for example, within a range of
200 angstroms-5000 angstroms. Such film thicknesses can be readily
achieved by the process and apparatus according to the invention.
The invention can also be used to generate much thicker films, if
desired.
FIG. 5 is a drawing of an electron micrograph of an actual device
fabricated according to the process of the invention utilizing the
apparatus of the invention. This drawing illustrates the step
coverage of a thin film 506 of strontium bismuth tantalate applied
to a substrate 5. Because of the relative thinness of the strontium
bismuth tantalate layer 506, it is not possible to show all the
details of the substrate 5 in a photomicrograph. Thus, a schematic
diagram intended to represent a cross-section of an integrated
circuit, is shown in FIG. 6. So that all the layers can be shown in
one drawing, the relative thicknesses of the various layers are not
drawn to the same scale in FIG. 6. As shown in FIG. 6, the
substrate 5 includes a silicon wafer 622, a layer 624 of SiO.sub.2
about 5000 .ANG. (Angstroms) thick, a layer 626 of titanium about
200 .ANG. thick, and a layer 628 of platinum about 2000 .ANG.
thick. In the actual capacitor, after the layer 506 of the layered
superlattice material is deposited, another approximately 2000
.ANG. thick layer 632 of platinum is deposited, then the capacitor
is patterned to complete the device.
As shown in FIG. 5, a step 508 is formed in substrate 5 over which
the layer 506 of strontium bismuth tantalate was deposited using
the method of the present invention. Note that the deposition of
the applied layered superlattice material 506 is extremely
conformal over the top 512 and bottom 514 of step 508. There is a
small amount of filling in of the hard angle near the bottom 514 of
the step 508, but this filling in is substantially less than for
layered superlattice materials deposited without
hexamethyl-disilazane as a solvent, and compares well to the
conformation possible in state-of-the-art integrated circuit
deposition techniques commonly used in the fabrication of
integrated circuits. Thus, the addition of hexamethyl-disilazane as
a solvent solves the problem of it not being possible to obtain the
excellent electronic properties of layered superlattice materials
in combination with the complex structures of state-of-the art
integrated circuits.
A dynamic random access memory (DRAM) cell 770 utilizing the
layered superlattice material is shown in FIG. 7. As is well-known,
a DRAM memory is made up of hundreds of thousands or millions of
such cells. Portions of the circuit wafer 850, particularly the
layer 860, may be formed utilizing the apparatus and process of the
invention. When the layer 860 is a ferroelectric layered
superlattice material, such as strontium bismuth tantalate, the
cell is a non-volatile ferroelectric (FERAM) switching memory cell,
and when the layer 860 is a dielectric layered superlattice
material, such as lead bismuth niobate, the cell 870 is a volatile
DRAM memory cell. The wafer 850 includes a silicon substrate 851,
field oxide areas 854, and two electrically interconnected
electrical devices, a transistor 871 and a capacitor 872.
Transistor 871 includes a gate 873, a source 874, and a drain 875.
Capacitor 872 includes first electrode 858, layered superlattice
material 860, and second electrode 877. Insulators, such as 856,
separate the devices 871, 872, except where drain 875 of transistor
871 is connected to first electrode 858 of capacitor 872.
Electrical contacts, such as 847 and 878 make electrical connection
to the devices 871, 872 to other parts of the integrated circuit
850. A detailed example of the complete fabrication process for an
integrated circuit memory cell as shown in FIG. 7 is given in U.S.
Pat. No. 5,466,629 issued Nov. 14, 1995. The detailed preferred
process for fabricating the layer 860 is given below. The process
of the invention discussed herein may also be utilized in forming
other layers of wafer 850, such as insulating layers 856.
A thin film deposition apparatus 1 according to one exemplary
embodiment of the invention is shown in FIGS. 1 and 1A. Apparatus 1
comprises a deposition chamber 2 containing a substrate holder 4, a
barrier plate 6, an input nozzle assembly 8, an exhaust nozzle
assembly 10, and an ultraviolet radiation source 16. The deposition
chamber 2 includes a main body 12, a lid 14 which is securable over
the main body 12 to define an enclosed space within the deposition
chamber 2. The chamber is connected to a plurality of external
vacuum sources which will not be described in detail herein. Lid 14
is pivotally connected to the main body 12 using a hinge as
indicated at 15. In operation, a mist and inert carrier gas are fed
from manifold assembly 40 (FIG. 3) via tube 45, in direction 43,
and pass to input nozzle assembly 8, where the mist is deposited
onto substrate 5. Excess mist and carrier gas are drawn out of
deposition chamber 2 via exhaust nozzle 10.
Substrate holder 4 is made from two circular plates 3, 3' of
electrically conductive material, such as stainless steel, the top
plate 3 being insulated from the bottom plate (field plate) 3' by
an electrically insulative material, such as delrin. In an
exemplary embodiment, utilizing a 4 inch diameter substrate,
substrate holder 4 is nominally 6 inches in diameter and supported
on a rotatable shaft 20 which is in turn connected to a motor 18 so
that holder 4 and substrate 5 may be rotated during a deposition
process. An insulating shaft 22 electrically insulates the
substrate holder 4 and substrate 5 supported thereon from the DC
voltage applied to the deposition chamber main body 12 so that a DC
bias can be created between the substrate holder 4 and barrier
plate 6 (via chamber main body 12). Such a DC bias may be utilized,
for example, for field-poling of thin films as they are being
deposited on the substrate 5. Insulating shaft 22 is connected to
shaft 20 and shaft 20' by couplings 21. Electrical source 102 is
operatively connected to main body 12 of deposition chamber 2 at
connection 108 by lead 106 and via feedthrough 23 to brass sleeve
25 by lead 104 to effect a DC bias between field plate 3' and
barrier plate 6.
Barrier plate 6 is made of an electrically conductive material such
as stainless steel, and is of sufficiently large size to extend
substantially over the substrate 5 in parallel thereto so that a
vaporized source or mist as injected through input tube 26 and
nozzle assembly 8 is forced to flow between barrier plate 6 and the
substrate holder 4 over the substrate 5. Barrier plate 6 is
preferably the same diameter as the substrate holder 4. The barrier
plate 6 is preferably connected to the lid 14 by a plurality of
rods 24 so that the plate 6 will be moved away from the substrate 5
whenever the lid is opened. The barrier plate 6 also includes a UV
transmitting window (not shown in FIG. 1).
The input nozzle assembly 8 and the exhaust nozzle assembly 10 are
more particularly shown with reference to FIG. 2. Input nozzle
assembly 8 includes an input tube 26 which receives a misted
solution from manifold assembly 40 as discussed below in relation
to FIG. 3. Input tube 26 is connected to arcuate tube 28 which has
a plurality of small holes or input ports 31 for accepting
removable screws 30 spaced 1/4 inch center-to-center along the
inner circumference of the tube 28.
Exhaust nozzle assembly 10 comprises an arcuate exhaust tube 29
having a plurality of small holes or exhaust ports 31' with
removable screws 30. The structure of the exhaust nozzle assembly
10 is substantially the same as that of the input nozzle assembly
8, except that a tube 34 leads to a vacuum/exhaust source (not
shown). End caps 32 of tubes 28 and 29 are removable for cleaning.
Arcuate tube 28 of input nozzle assembly 8 and the corresponding
arcuate tube 29 of exhaust nozzle assembly 10 respectively surround
oppositely disposed peripheral portions 4-1, 4-2 of substrate
holder 4.
In an exemplary embodiment wherein a layered superlattice material
film is to be deposited, the centers of holes 31, 31' in tubes 28
and 29 are nominally located 0.375 inches above substrate holder 4.
However, referring to FIG. 1, this distance is adjustable using
different lengths of shaft 20' to suit the specific deposition
process.
Each of the tubes 28, 29, is typically fabricated from 1/4" O.D.
stainless steel, with an inner diameter of approximately 3/16". The
interior walls of each tube 28,29 are preferably electro-polished.
Holes 31, 31' in tubes 28 and 29 respectively are spaced
approximately 1/4" center-to-center, and are tapped to accommodate
4-40 (1/8") socket head set screws.
Through such structure, and by adjusting the location of open holes
31, 31' by selectively inserting or removing screws 30 in the two
arcuate tubes 28 and 29, the flow of a vaporized solution or mist
over the substrate 5 can be well controlled for various solutions
and flow rates, etc., to achieve a uniform deposition of a thin
film on substrate 5.
Referring to FIGS. 1 and 2, substrate holder 4, barrier plate 6,
input nozzle assembly 8 and exhaust nozzle assembly 10 collectively
cooperate to define a relatively small, semi-enclosed deposition
area 17 surrounding an upper/exposed surface of the substrate 5,
and within which the vaporized solution is substantially contained
throughout the deposition process.
Although exemplary embodiments of substrate holder 4, barrier plate
6, input nozzle assembly 8 and exhaust nozzle assembly 10 are shown
and described, it is understood that variations of such structures
can be utilized within the scope of the present invention. For
example, the arcuate input and exhaust tubes 28 and 29 could be
replaced with tubes of other structures such as V-shaped or
U-shaped tubes, or slotted tubes, or could simply be replaced by a
plurality of separate nozzles and separate exhaust ports.
FIG. 3 shows a manifold assembly 40 according to the present
invention. The manifold assembly 40 is utilized for supplying a
mist to input nozzle assembly 8, and generally comprises a mixing
chamber 42, a plurality of inlets 44 which are connected to
corresponding mist generators 46-1, 46-2, through 46-n through
respective valves 49-1, 49-2, through 49-n, a deposit valve 47 for
regulating flow from the mixing chamber 42 to the nozzle assembly
8, and an exhaust vent valve 48. In use, one or more of the mist
generators 46-* are utilized to generate one or more different
mists which are then flowed into the mixing chamber 42 through
valves 49-* and inlets 44.
The mists as flowed into the mixing chamber 42 are mixed to form a
single, uniform misted solution which is then flowed into the
deposition chamber 2 at an appropriate flow rate through the valve
47 and input tube 26. The general direction of flow in the mixing
chamber 42 and tube 45 which connects manifold assembly 40 to input
nozzle assembly 8 (FIG. 1) is shown by the arrow 43. Valve 47 can
be selectively closed off so that the deposition chamber 2 can be
pumped down if desired, or to clean and purge the manifold system
when necessary. Similarly, the outlet 51 of the exhaust valve 48 is
connected to a vacuum source (not shown) so that, when necessary to
exhaust/purge one or more of the mist generators 46, valve 47 can
be closed off, valve 48 and one or more of the valves 49-* can be
opened, and the mixing chamber 42 can be pumped down to clean and
purge the mist generator(s) 46-* and the mixing chamber 42 by
applying a vacuum via outlet 51, or using standard negative draw
type exhaust.
Apparatus 1 shown in FIGS. 1, 7, and 9 includes electrical means
102 for applying a DC bias in the deposition chamber 2 during a
deposition operation. FIG. 1 shows the DC input 104. The DC
potential applied between input sleeve 25 and deposition chamber
main body 12 is typically 350 volts. The DC bias achieves poling
in-situ of the ferroelectric film adding to the film quality.
Dipole ordering along the crystal c-axis (the major polarization
axis) is often desirable, and the resulting ordering reduces
dislocation density which can be responsible for fatigue and
retention problems. A DC bias of either greater than or less than
350 volts could also be used to effectuate the above results. In
addition, while deposition is occurring, combinations of
ultraviolet radiation and DC bias may be applied within chamber 2
either together or sequentially, and repeated.
The above details or the apparatus 1 are sufficient to understand
the process of the invention. Further details may be found in U.S.
Pat. No. 5,456,945 issued Oct. 10, 1995.
2. Detailed Description of the Process
Referring to FIG. 4, there is shown an exemplary flow chart
depicting the fabrication of a layered superlattice material thin
film according to the invention. In steps P1 through P6 the liquid
precursor is made. The process shown in the preferred process for
fabricating a layered superlattice material in which there are
three metallic elements. In the preferred embodiment, in each of
steps P1 through P3 an initial precursor is made by reacting a
metal or metal compound with a carboxylic acid to form a metal
carboxylate, which is dissolved in a solvent. That is, in this
embodiment, the metal moiety is a metal carboxylate. The preferred
carboxylic acid for the reaction is one having a medium-length
ligand, such as 2-ethylhexanoic acid, although others may be used.
Preferably the solvent's boiling point should be within the range
110.degree. C.-170.degree. C. The preferred solvents are alcohols,
such as 2-methoxyethanol, aromatic hydrocarbons, such as the
xylenes and octane, and esters, such as n-butyl acetate, though any
of the solvents in Table A may be used.
TABLE A ______________________________________ Solvent Boiling
Point ______________________________________ xylenes (bp =
138.degree. C.-143.degree. C.) n-Butyl acetate (bp = 126.5.degree.
C.) octane N,N-dimethylformamide (bp = 153.degree. C.)
2-Methoxyethyl acetate (bp = 145.degree. C.) Methyl isobutyl ketone
(bp = 116.degree. C.) Methyl isoamyl ketone (bp = 144.degree. C.)
Isoamyl alcohol (bp = 132.degree. C.) Cyclohexanone (bp =
156.degree. C.) 2-Ethoxyethanol (bp = 135.degree. C.)
2-Methoxyethyl ether (bp = 162.degree. C.) Methyl butyl ketone (bp
= 127.degree. C.) Hexyl alcohol (bp = 157.degree. C.) 2-Pentanol
(bp = 119.degree. C.) Ethyl butyrate (bp = 121.degree. C.)
Nitroethane (bp = 114.degree. C.) Pyrimidine (bp = 123.degree. C.)
1,3,5 Trioxane (bp = 115.degree. C.) Isobutyl isobutyrate (bp =
147.degree. C.) Isobutyl propionate (bp = 137.degree. C.) Propyl
propionate (bp = 122.degree. C.) Ethyl Lactate (bp = 154.degree.
C.) n-Butanol (bp = 117.degree. C.) n-Pentanol (bp = 138.degree.
C.) 3-Pentanol (bp = 116.degree. C.)
______________________________________
The amounts of the metals used are usually proportioned so that an
equivalent weight of each metal equal to the molecular weight of
the metal in the stoichiometric formula for the desired layered
superlattice material. An exception is lead. Generally an excess of
lead of between 1% and 100%, preferably between 3% and 10%, of the
equivalent stoichiometric amount is included because lead oxide has
a higher vapor pressure than the other metals and a significant
amount of lead evaporates as lead oxide during baking and
annealing. Similarly, excess amounts of other materials, such as
bismuth, thallium, and antimony, that evaporate or otherwise are
lost in the process may be used. For bismuth excellent results have
been obtained with from about 2% to 70% excess bismuth, with the
best results being in the range of 10% to 40% excess, although this
factor is strongly dependent on the details of the heating steps
P11 and P12.
If it is desired to add dopants to the material, then initial
precursor(s) of the dopant element(s) may be made in step P5 in a
similar manner to the precursors made in steps P1-P3.
Alternatively, the dopant(s) may be added in the mixing step
P6.
The steps P1, P2, P3, and P4 are preferably performed by mixing the
metal or other metal compound, such as a metal alkoxide, with the
carboxylic acid and the solvent and stirring. Low heat of between
about 70.degree. C. and 90.degree. C. may be added to assist the
reaction and dissolving, but this is generally not necessary. The
solid arrows indicate that generally, the chemist will perform all
the steps P1, P2 and P3 in the same container; that is the first
metal or metal compound, the first measure of carboxylic acid, and
a first solvent are placed in a container, the metal or metal
compound and carboxylic acid are reacted, and the reactant
dissolved, the second metal or metal compound is then placed in the
same container and additional carboxylic acid and solvent are added
and stirred to react the second metal or metal alkoxide and
dissolve the reactant, then the third metal or metal compound,
third carboxylic acid, and third solvent are added, the metal or
metal compound is reacted, and the reactant dissolved. In this
process the most reactive metal or metal compound is preferably
added first, the second most reactive metal or metal compound added
second, and the least reactive metal or metal compound added last.
It also may be desirable to perform the distillation step after
each or some of the metal and/or metal compounds are reacted and
dissolved. Alternatively, each metal and/or metal compound may be
combined with a carboxylic acid and solvent, reacted, and dissolved
in a separate container, the result distilled if desirable, and
then the three separate solutions mixed in step P6. Variations and
combinations of preparing the individual metal precursors
separately or in the same container with or without distillation(s)
may be used depending on the solvents used and the form in which
metal element is readily available. In addition it should be
understood that if the desired superlattice material includes only
two metallic elements, then only two metals or metal compounds will
be reacted and dissolved, and if the desired superlattice material
includes four or more metallic elements, then four or more metals
or metal compounds will be reacted and dissolved. Further, it is
understood that any of the steps P1, P2, and P3 may be replaced by
using a preprepared metal carboxylate. In addition, many other
processes for preparing the initial precursor may be used, as for
example the variations discussed in U.S. Pat. No. 5,468,679 issued
Nov. 21, 1995.
When the solution of reacted and dissolved metal carboxylates has
been prepared, the precursor solution is then mixed and distilled
in step P6 by heating and stirring the solution to further the
reaction of the reagents, reduce the solution to the desired volume
and viscosity, which may depend on whether the solution is to be
stored or used immediately, and/or to remove certain liquids, such
as water. Generally, if it is desirable to remove certain liquids,
the solution is heated to a temperature above the boiling point of
the liquids to be removed and below the boiling point of the
liquids that are desired to be retained. The solution is distilled
until all the solvents that are desired to be removed have
evaporated and a desired volume and viscosity are reached. It may
be necessary to add the desired solvent several times in the
distilling process to remove all undesired solvents and obtain the
desired volume and viscosity. Preferably, as much water as possible
is distilled out so that the resulting initial precursor is
essentially anhydrous.
Optionally, either separately or in combination with the step P6, a
solvent exchange step may be performed. In this step a solvent,
such as xylene, is added and the other solvents are distilled away.
This solvent exchange step may be performed as the final step in
preparation of the precursor prior to storing to change to a
solvent that stores well, and or just before the misting step P9 to
change to a solvent that deposits well, or both. If it is known
that a certain solvent, such as xylene, will be preferable, the
solvent may be added with the other solvents in steps P1, P2, P3,
P4 and/or P5 and the other solvents distilled away in the
distillation step P6.
Just before forming the mist, in step P8, an initiator may be added
to the precursor. An initiator is a high vapor pressure, low
boiling point, solvent that assists in initiating the formation of
the mist. Preferably, the metal moieties in the precursor are also
soluble in the initiator, that is, the initiator is a solvent for
the metal moieties. A liquid with a boiling point of between about
50.degree. C. and 100.degree. C. is preferred as an initiator.
Examples of liquids that may be used as initiators are given in
Table B.
TABLE B ______________________________________ Initiator Boiling
Point ______________________________________ Methyl Ethyl Ketone
(2-butanone) 80.degree. C. Isopropanol 82.degree. C. Methanol
64.7.degree. C. Tetrahydrofuran 67.degree.0 C.
______________________________________
Examples of the preparation of layered superlattice precursor
solutions are given in U.S. Pat. No. 5,423,285 issued Jun. 13,
1995. Precursors for layered superlattice materials made by the
processes described above are now commercially available from
Kujundo Chemical Laboratory Co. Ltd. (KJC), No. 1-28 5 Chome,
Chiyoda, Sakado-shi, Saitama pref., Japan.
EXAMPLE 1
In the preferred process, 6.5 milliliters (ml) of KJC-Strontium
Bismuth Tantalate-34611 F solution, including bismuth, strontium
and tantalum 2-ethylhexanoates in xylenes, was mixed with 5.5 ml of
methyl ethyl ketone (MEK), to which was added 2 ml of
hexamethyl-disilazane. This final precursor solution was placed in
mist generator 46-1. The deposition chamber 12 was pumped down to
10.sup.-6 Torr. Substrate rotation motor 18 was turned on to rotate
substrate holder 4 at 60 cycles a minute. UV source 16 was then
turned on for 30 minutes to desorb the moisture in the deposition
chamber as well as any moisture on the substrate. The deposition
chamber was slowly back filled with an inert nitrogen gas to a
pressure of approximately 595 Torr. Next, the process vacuum line
was opened to stabilize the deposition chamber pressure at
approximately 595 Torr. In step P9 deposit valves 49-1 and 47 were
opened, argon gas was passed through ultrasonic mist generator 46,
and the mist generator was then turned on to form the mist. In step
P10, the mist was flowed into the deposition chamber 12 for 9
minutes and deposited on a substrate 5 comprising a silicon wafer
622 with layers of silicon dioxide 624, titanium 626, and platinum
628 deposited on it. The UV source 16 was left on through this
process. The wafer 600 was removed from the deposition chamber 12
and placed on a hot plate where it was dried in step P11 at a
temperature of 150.degree. C. for one minute, then processed by
rapid thermal anneal (RTA) at 725.degree. C. for 30 seconds in
oxygen in step P12. The first layer thickness was 900 .ANG.. Then
the wafer 600 was returned to the deposition chamber 12, the mist
was formed again and steps P9 and P10 another layer was deposited
for six minutes. The wafer was then removed and baked at
260.degree. C. for four minutes, and again RTA processed at
725.degree. C. for 30 seconds in oxygen, and then, in step P13,
annealed in oxygen for one hour. The resulting film 506 was
approximately 1400 Angstroms (.ANG.) thick.
At the end of each of the two coating processes, the mist generator
46-1, UV source 16, and substrate rotation motor 18 were turned
off, deposit valve 47 was closed, mist generator 46-1 was turned
off, and manifold 40 was vented until mist generator 46-1 reached
ambient temperature. At the end of the entire deposition process,
manifold 42 was purged through vent 705 with argon gas.
After the anneal step P13, the IC device 600 was completed in step
P14, i.e. second platinum electrode 632 was sputtered on and the
wafer was then etched using well-known photo-resist techniques to
produce a plurality of capacitors 604.
Hysteresis measurements were made on the strontium bismuth
tantalate capacitor fabricated by the above process using an
uncompensated Sawyer-Tower circuit at 10,000 Hertz and at voltages
of 1 volt, 3 volts, 4 volts, 5 volts, 6 volts, 7 volts, 8 volts, 9
volts, and 10 volts. The results for room temperature are shown in
FIG. 8. The ordinate is the polarization in microcoulombs per
square centimeter while the abscissa is the applied electric field
in kilovolts per centimeter. The hysteresis curves indicate the
capacitors would perform well in a memory. Almost all the curves
except the 1 volt curve fall one on top of the other, showing that
the electrical properties remain remarkably constant over the range
of voltages common to integrated circuits. The polarizability, 2Pr,
is over 19 microcoulombs/cm.sup.2 for the 5 volt measurement and
about the same for all voltages from 3 to 10 volts. The coercive
field, 2Ec, was about 85 volts for the 5 volt measurement, and
about the same for all voltages from 3 to 10 volts.
FIG. 9 shows a graph of the remnant polarizations, +Pr and -Pr, in
microcoulombs per centimeter squared versus the number of switching
cycles for the sample made by the process described above, also at
room temperature. The measurements were made at 10,000 and a
million hertz with an amplitude of 5 volts using a triangular wave.
Both curves are essentially flat out to 10.sup.9 cycles. These
curves are generally referred to in the art as fatigue curves,
since any decrease in these curves indicates that the ferroelectric
properties is declining with increasing number of switching cycles,
which is referred to as fatigue of the ferroelectric material.
Prior to the discovery of the extraordinary properties of the
layered superlattice materials by some of the present inventors,
all known ferroelectric materials fatigued 50% or more over the
range shown in the graph. The graph shows that strontium bismuth
tantalate made by the process of the invention retains the same
resistance to fatigue while also having good step coverage.
The measured leakage current at room temperature in amperes per
square centimeter versus applied voltage in volts for the same
sample is shown in FIG. 10. The leakage current stays well below
10.sup.-6 amps per square centimeter for all measured voltages.
FIG. 5, discussed above, is a drawing of an electron micrograph of
an actual layer of strontium bismuth tantalate deposited on a
platinum substrate as described in Example 1.
The above results are all excellent, and are comparable to the best
results obtained for strontium bismuth tantalate for flat
capacitors. Thus, the use of hexamethyl-disilazane provides
excellent step coverage while retaining extraordinary electrical
properties.
EXAMPLE 2
As a check on the results, a second set of samples was made using
the same process described above, except that the
hexamethyl-disilazane was omitted. All samples showed blue spots,
which upon magnification proved to be thickness variations. The
Polarizability, 2Pr, was typically about 14 microcoulombs per
centimeter squared, although a few showed results the same as those
given above, indicative of variations in coverage of the substrate
by the superlattice material. Photomicrographs confirmed the poor
step coverage. While variations of the precursor and other portions
of the process could be found that produced good electrical
results, no variation, other than the use of hexamethyl-disilazane
could be found that produced uniform thickness and good step
coverage.
EXAMPLE 3
A third set of samples was made using the same process as described
in Example 1 except that a strontium bismuth tantalum niobate
solution was used. The step coverage was essentially the same as
for the strontium bismuth tantalum capacitor samples of Example 1.
Because of the excellent appearance, it is expected that the
electronic properties will be likewise excellent, though these have
not yet been measured at this time.
Although there has been described what is at present considered to
be the preferred embodiments of the present invention, it will be
understood that the invention can be embodied in other specific
forms without departing from the spirit or essential
characteristics thereof. Now that the advantage of using
hexamethyl-disilazane in the above liquid precursor deposition
process has been disclosed, the solvent may be found useful for
other liquid deposition processes. The present embodiments are,
therefore, to be considered as illustrative and not restrictive.
The scope of the invention is indicated by the appended claims.
* * * * *